While not limited thereto, the present invention is particularly adapted for use with optical laser interferometer apparatus wherein a light beam from a laser passes through a beam splitter, with a portion of the beam passing to a retroreflector mounted on a member movable with respect to the beam splitter. The reflected beam is then applied to an interferometer which will produce, through interference phenomena or the Doppler effect, a number of output pulses proportional to the travel of the member on which the retroreflector is mounted with respect to a fixed point. Such devices have utility, for example, in machine tools where the travel of one or more workpiece support tables must be measured.
In applications of this sort, it is essential to completely enclose the retroreflector and laser beam for all measurements of the member on which the movable retroreflector is mounted. In this respect, it is essential to reduce or eliminate contamination of the laser optics and/or environmental effects on the laser light. Oil-mist, dust, moisture and other materials present in the atmosphere have a tendency to collect on exposed optical surfaces, thus reducing the effective reflected or transmitted laser light energy. In addition, air temperature gradients, which cause the laser beam to move spatially about its line of sight, tend to reduce the effectiveness and reliability of the laser measuring device. The solution is to enclose the optics in an enclosure and to enclose the laser light beam in a flight tube.
A flight tube of this type must be sufficiently rigid to maintain a straight-line orientation when mounted in a horizontal attitude. In the past, rigid tubing has been employed as a support mechanism with one end (usually the end closest to the laser optics) held rigid. The other end is permitted to slide through a hole in the retroreflector support bracket. The problem with this technique, however, is that when the flight tube length is a minimum, the excess support tubing length extends beyond the end of a machine or the like on which it is used, thus causing a hazardous condition and wasted space. Also, rigid support tubes have a tendency to droop downwardly or bend when the flight tube is at a maximum length, thus interfering with the laser beam itself.
Theoretically, the problems encountered with rigid flight tubes comprising one rigid tube which slides within another can be obviated by means of a flexible bellows. The ordinary flexible bellows, however, will sag and cannot be maintained straight when it is extended.